Esters of polyhydric alcohols, also known as polyol esters, find a wide range of varying uses in industry, for example as plasticizers or lubricants. The selection of suitable starting materials allows the physical properties, for example boiling point or viscosity, to be controlled, and the chemical properties, such as hydrolysis resistance or stability to oxidative degradation, to be taken into account. Polyol esters can also be tailored to the solution of specific performance problems. Detailed overviews of the use of polyol esters can be found, for example, in Ullmann's Encyclopedia of Industrial Chemistry, 5th edition, 1985, VCH Verlagsgesellschaft, vol. A1, pages 305-319; 1990, vol. A15, pages 438-440, or in Kirk Othmer, Encyclopedia of Chemical Technology, 3rd edition, John Wiley & Sons, 1978, vol. 1, pages 778-787; 1981, vol. 14, pages 496-498.
The use of polyol esters as lubricants is of great industrial significance, and they are used particularly for those fields of use in which mineral oil-based lubricants only incompletely meet the requirements set. Polyol esters are used especially as turbine engine and instrument oils. Polyol esters for lubricant applications are based frequently on 1,3-propanediol, 1,3-butanediol, diethylene glycol, triethylene glycol, tetraethylene glycol, 1,2-propylene glycol or higher propylene glycols. They can be prepared in different ways. In addition to the reaction of alcohol and acid, optionally in the presence of acidic catalysts, further processes are employed in practice to obtain G esters, including the reaction of diol with acid halide, the transesterification of a carboxylic ester with a diol, and the addition of ethylene oxide onto carboxylic acids (ethoxylation). In industrial manufacture, only the direct reaction of diol and carboxylic acid and the ethoxylation of carboxylic acids have become established as production processes, preference usually being given to the esterification of diol and acid. This is because this process can be performed with no particular complexity in conventional chemical apparatus, and it affords chemically homogeneous products. Compared to this, ethoxylation requires extensive and costly technical equipment. Ethylene oxide is a very reactive chemical substance. It can polymerize explosively and forms explosive mixtures with air within very wide mixing ranges. Ethylene oxide irritates the eyes and the respiratory tract, leads to chemical burns and to liver and kidney damage, and is carcinogenic. The handling thereof therefore entails extensive safety measures. Moreover, scrupulous cleanliness of storage apparatus and reaction apparatus has to be ensured, in order to rule out the formation of undesired impurities as a result of side reactions of the ethylene oxide with extraneous substances. Finally, the reaction with ethylene oxide is not very selective, since it leads to mixtures of compounds of different chain length.
The direct esterification of alcohols with carboxylic acids is one of the basic operations in organic chemistry. In order to increase the reaction rate, the conversion is typically performed in the presence of catalysts. The use of one of the reactants in excess and/or the removal of the water formed in the course of the reaction ensures that the equilibrium is shifted in accordance with the law of mass action to the side of the reaction product, i.e. of the ester, which means that high yields are achieved.
Comprehensive information regarding the preparation of esters of polyhydric alcohols, also including esters of ethylene glycols and fatty acids, and regarding the properties of selected representatives of these compound classes can be found in Goldsmith, Polyhydric Alcohol Esters of Fatty Acids, Chem. Rev. 33, 257 ff. (1943). For example, esters of diethylene glycol, of triethylene glycol and of polyethylene glycol are prepared at temperatures of 130 to 230° C. over reaction times of 2.5 to 8 hours. To remove the water of reaction, carbon dioxide is used. Suitable catalysts mentioned for the esterification of polyhydric alcohols are inorganic acids, acidic salts, organic sulphonic acids, acetyl chloride, metals or amphoteric metal oxides. The water of reaction is removed with the aid of an entraining agent, for example toluene or xylene, or by introducing inert gases such as carbon dioxide or nitrogen.
The production and the properties of fatty acid esters of the polyethylene glycols are discussed by Johnson (edit.), Fatty Acids in Industry (1989) Chapter 9, Polyoxyethylene Esters of Fatty Acids, and a series of preparative hints are given. Higher diester concentrations are achieved by the increase in the molar ratio of carboxylic acid to glycol. Suitable measures for removing the water of reaction are azeotropic distillation in the presence of a water-immiscible solvent, heating while passing through an inert gas, or performing the reaction under reduced pressure in the presence of a desiccant. When the addition of catalysts is dispensed with, longer reaction times and higher reaction temperatures are required. Both reaction conditions can be made milder by the use of catalysts. In addition to sulphuric acid, organic acids such as p-toluenesulphonic acid and cation exchangers of the polystyrene type are the preferred catalysts. The use of metal powders, such as tin or iron, is also described. According to the teaching from U.S. Pat. No. 2,628,249, colour problems in the case of catalysis with sulphuric acid or sulphonic acid can be alleviated when working in the presence of activated carbon.
Further metallic catalysts used to prepare polyol esters are also alkoxylates, carboxylates or chelates of titanium, zirconium or tin, for example according to U.S. Pat. No. 5,324,853 A1. Such metal catalysts can be considered as high-temperature catalysts, since they achieve their full activity only at high esterification temperatures, generally above 180° C. They are frequently added not at the start of the esterification reaction, but after the reaction mixture has already been heated up and has reacted partly with elimination of water. In spite of the relatively high reaction temperatures and relatively long reaction times required compared to the conventional sulphuric acid catalysis, crude esters with a comparatively low colour number are obtained in the case of catalysis with such metal compounds. Common esterification catalysts are, for example, tetra(isopropyl) orthotitanate, tetra(butyl) orthotitanate, tetra(butyl) zirconate or tin(II) 2-ethylhexanoate.
The catalytic esterification reaction of polyols with carboxylic acids achieves, based on the component present in deficiency, a high conversion within a comparatively short time, but a comparatively long reaction time has to be accepted for the remaining conversion to the desired polyol esters. Although a procedure hydrolyses the metal compounds to insoluble solids, which can be filtered off before the further workup of the crude ester compound. According to U.S. Pat. No. 4,304,925 A1, the crude esterification product, before addition of alkali, is first admixed with water and treated under hot conditions. This converts the hydrolysed metal compounds to readily filterable precipitates.
U.S. Pat. No. 2,628,249 A discloses the esterification of ether polyols with aliphatic monocarboxylic acids. Color problems in the case of catalysis with sulphuric acid or sulphonic acids can be alleviated when the esterification is performed in the presence of activated carbon.
The prior art for preparation of polyol esters under metal catalysis requires either a special reactor design in order to complete the esterification reaction within an economically acceptable time, or an additional treatment with water under hot conditions, in order to substantially completely remove the metallic catalyst after the esterification reaction has ended with formation of hydrolysis products which can be filtered off readily.
Even though polyol esters are generally obtained with satisfactory colour number in the case of use of metallic catalysts, industrial production occasionally also gives products which do not meet the specification values with regard to colour number and acid number. While the process according to DE 10 2009 048 775 A1 enables polyol esters to be obtained in high quality in a simple manner, it is desirable to provide a process in which a simple aftertreatment affords on-spec polyol esters if the polyol esters obtained by the production process according to DE 10 2009 048 775 A1 should not have the required specification, for example due to a fault which occurs during industrial production.
EP 2 308 821 A2 discloses a process for lightening the color of polyol esters, wherein the reaction mixture, in the course of workup, after removal of unconverted starting compounds, is treated with ozone or ozone-containing gases, immediately followed without any further intervening steps by a steam treatment. The workup of the reaction mixture is conducted without adsorbents.
EP 2 308 822 A2 considers an analogous process for lightening the color of polyol esters using peroxidic compounds.
DE 27 29 627 A1 also discloses the treatment of carboxylic esters with ozone. After the ozone treatment, the reaction mixture is neutralized with an aqueous alkali solution and washed with water. Volatile constituents are subsequently driven out at elevated temperature and under reduced pressure or at standard pressure. According to the process from DD 57 596 A, aromatic dicarboxylic esters, to lighten the color, are admixed with an aqueous hydrogen peroxide solution in the presence of alkali and then subjected to a steam treatment.
DE 101 21 866 A1 discloses a transesterification process for preparing fatty acid polyol esters, which is conducted in the presence of reducing agents and alkali metal bases. The transesterification reaction may be followed by bleaching with hydrogen peroxide. DE 197 41 913 C1 proposes admixing a reaction product which is obtained by esterification of fatty acids with alcohols under Sn4+ catalysis, in the course of workup, with a combined reducing agent and precipitant. This forms sparingly soluble Sn2+ compounds.
It was therefore an object of the present invention to provide a process in which a simple aftertreatment can improve the quality of polyol esters already prepared and worked up under metal catalysis such that on-spec polyol esters which can be used in a wide variety of ways are obtained.